1 /*!
2 @file
3 Defines `boost::hana::detail::unpack_flatten`.
4 
5 @copyright Louis Dionne 2013-2017
6 Distributed under the Boost Software License, Version 1.0.
7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
8  */
9 
10 #ifndef BOOST_HANA_DETAIL_UNPACK_FLATTEN_HPP
11 #define BOOST_HANA_DETAIL_UNPACK_FLATTEN_HPP
12 
13 #include <boost/hana/at.hpp>
14 #include <boost/hana/config.hpp>
15 #include <boost/hana/detail/algorithm.hpp>
16 #include <boost/hana/detail/array.hpp>
17 #include <boost/hana/length.hpp>
18 #include <boost/hana/unpack.hpp>
19 
20 #include <cstddef>
21 #include <utility>
22 
23 
24 BOOST_HANA_NAMESPACE_BEGIN namespace detail {
25     template <std::size_t ...Lengths>
26     struct flatten_indices {
27         // avoid empty arrays by appending 0 to `lengths`
28         static constexpr std::size_t lengths[sizeof...(Lengths) + 1] = {Lengths..., 0};
29         static constexpr auto flat_length =
30             detail::accumulate(lengths, lengths + sizeof...(Lengths), 0);
31 
32         template <bool Inner>
computedetail::flatten_indices33         static constexpr auto compute() {
34             detail::array<std::size_t, flat_length> indices{};
35             for (std::size_t index = 0, i = 0; i < sizeof...(Lengths); ++i)
36                 for (std::size_t j = 0; j < lengths[i]; ++j, ++index)
37                     indices[index] = (Inner ? i : j);
38             return indices;
39         }
40 
41         static constexpr auto inner = compute<true>();
42         static constexpr auto outer = compute<false>();
43 
44         template <typename Xs, typename F, std::size_t ...i>
45         static constexpr decltype(auto)
applydetail::flatten_indices46         apply(Xs&& xs, F&& f, std::index_sequence<i...>) {
47             return static_cast<F&&>(f)(
48                 hana::at_c<outer[i]>(hana::at_c<inner[i]>(
49                     static_cast<Xs&&>(xs)
50                 ))...
51             );
52         }
53     };
54 
55     struct make_flatten_indices {
56         template <typename ...Xs>
57         auto operator()(Xs const& ...xs) const -> detail::flatten_indices<
58             decltype(hana::length(xs))::value...
59         >;
60     };
61 
62     template <typename Xs, typename F>
unpack_flatten(Xs && xs,F && f)63     constexpr decltype(auto) unpack_flatten(Xs&& xs, F&& f) {
64         using Indices = decltype(hana::unpack(xs, make_flatten_indices{}));
65         return Indices::apply(static_cast<Xs&&>(xs), static_cast<F&&>(f),
66                         std::make_index_sequence<Indices::flat_length>{});
67     }
68 } BOOST_HANA_NAMESPACE_END
69 
70 #endif // !BOOST_HANA_DETAIL_UNPACK_FLATTEN_HPP
71